Processing and Application of Ceramics 12 [1] (2018) 86–93

https://doi.org/10.2298/PAC1801086H

A self-setting particle-stabilized porous ceramic panel prepared from

commercial cement and loaded with carbon for potential radar-

absorbing applications

Jang-Hoon Ha1,∗, Sujin Lee1, Jae Ryung Choi1, Jongman Lee1, In-Hyuck Song1,Tai-Joo Chung2

1Powder and Ceramics Division, Korea Institute of Materials Science, 797 Changwondaero, Seongsan-gu,

Changwon, Gyeongnam 51508, Republic of Korea2School of Materials Science and Engineering, Industrial Technology Center for Environment-friendly

Materials, Andong National University, 1375 Gyeongdong-ro, Andong, Gyeongsangbuk-do 36729,

Republic of Korea

Received 25 August 2017; Received in revised form 12 December 2017; Received in revised form 10 February 2018;

Accepted 21 March 2018

Abstract

Porous ceramic materials are in a current research focus because of their outstanding thermal stability, chem-ical stability and lightweight. Recent research has widened the range of applications to radar absorption toutilize the advantages of porous ceramic materials. There has been long-standing interest in the developmentof lightweight radar-absorbing materials for military applications such as camouflaging ground-based facil-ities against airborne radar detection. Therefore, in this study, a novel lightweight radar-absorbing materialfor X-band frequencies was developed using a self-setting particle-stabilized porous ceramic panel compos-ited with carbon. The panel was prepared using a commercial calcium aluminate cement (as a self-settingmatrix), zeolite 13X particles with propyl gallate (as a particle-stabilized pore former) and carbon (as aradar-absorbing material). The panel contained macropores approximately 200 to 400 µm in size formed byzeolite 13X particles that are irreversibly adsorbed at liquid-gas interfaces. The self-setting particle-stabilizedporous ceramic panels were characterized by scanning electron microscopy, mercury porosimetry, physisorp-tion analysis, capillary flow porosimetry and network analysis. When 0.2 wt.% carbon was added to a self-setting particle-stabilized porous ceramic panel to fabricate a composite 7 mm thick, the maximum reflectionloss was −11.16 dB at 12.4 GHz. The effects of the amount of added carbon and the thickness variation ofa self-setting particle-stabilized porous ceramic panel on the radar-absorbing properties remain importantissues for further research.

Keywords: self-setting particle-stabilized porous ceramic panel, radar-absorbing properties

I. Introduction

Porous ceramics have received recent attention [1]

because of their unique properties, such as low density

[2], low thermal conductivity [3,4] and low dielectric

constant [5]. Among many applications of porous ce-

ramics, a porous ceramic panel for radar-absorbing ap-

plications is a feasible candidate. This is because con-

∗Corresponding author: tel: +82 55 280 3350,

e-mail: hjhoon@kims.re.kr

ventional porous polymer panels that are used for radar-

absorption applications (usually polyurethane foam)

cannot be operated under high temperature and harsh

environments owing to the limitations associated with

polymer materials.

Radar-absorbing materials are specially designed and

shaped to effectively absorb incident radio frequency

(RF) radiation from as many incident directions as pos-

sible. Since World War II, radar-absorbing materials

have attracted much attention because of their stealth

characteristics in military applications such as aircrafts,

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J.-H. Ha et al. / Processing and Application of Ceramics 12 [1] (2018) 86–93

land vehicles, naval vessels, and support facilities [6].

However, it is still very challenging work to develop a

low-cost, lightweight radar absorbing material. There-

fore, a few researchers have investigated low-cost mate-

rials such as electronic waste [7], rice husks [8] and for-

est fire ash [9]. Radar-absorbing materials can be pre-

pared by introducing conductive particles such as car-

bon or metal particles to induce a dielectric loss by

enhancing the conductivity of the material. In particu-

lar, carbon-based conductive particles have been widely

used because of their good absorption performance in

the high frequency bands [10]. These include carbon

nanotube-based composites [11,12] and carbon-based

composites [13,14]. Because porous ceramics are both

inherently lightweight and inexpensive, it is worthwhile

to study them as low-cost lightweight radar absorbing

materials.

If a radar-absorbing material needs to be impregnated

or adsorbed onto a porous ceramic matrix, the porous

ceramic matrix should have an open-cell pore struc-

ture. In the impregnation method [15], a highly open-

cell pore structure is soaked in a slurry consisting of

both a radar-absorbing material and a binder, until the

internal pores are filled in with the radar absorbing ma-

terial. However, in general, the mechanical strength of

an open-cell pore structure is significantly lower than

that of a closed-cell pore structure because of the in-

herent limitations of the replica technique, which is

mainly used when an open-cell pore structure is fabri-

cated [2,16]. The minimum pore size is also limited to

approximately 200µm, because of the difficulty of im-

pregnating an open-cell pore structure with excessively

narrow cells [1].

However, if a radar-absorbing material is composited

with a pore structure from the beginning of the prepa-

ration of a porous ceramic panel, the panel does not

need to be an open-cell pore structure, which has an in-

ferior mechanical strength, because no post-processing

such as impregnation or coating is required. Hence, in

this study, a particle-stabilized direct foaming method

was introduced in order to have a porous ceramic panel

composited with a radar-absorbing material from the

start. With the rapid progress in colloidal chemistry, a

closed-pore structure can now be fabricated by incorpo-

rating air into an aqueous suspension containing surfac-

tant molecules, then drying and sintering the resulting

sample. This processing method is known as a particle-

stabilized direct foaming method [16–27] and it has

been extensively studied as a promising route for fab-

ricating porous materials because of its exceptional sta-

bility. Particles that are irreversibly adsorbed at a liquid-

gas interface impede the coalescence or shrinkage of

bubbles, stabilize the liquid foam, and do not assem-

ble to yield aggregates, in the same way that surfactant

molecules form micelles. This processing method offers

an easy, inexpensive and rapid approach to fabricating

highly porous ceramics with a closed-cell pore struc-

ture.

Carbon as a radar-absorbing material cannot with-

stand high temperatures because oxidation reactions oc-

cur, which means that a conventional sintering process

is not appropriate for preparing a porous ceramic panel

composited with carbon. Therefore, pore forming by a

particle-stabilized method and self-setting by calcium

aluminate cement should provide an easy, inexpensive

and rapid approach to fabricating porous ceramic panels

composited with carbon. In addition, because the parti-

cle shapes of calcium aluminate cement and zeolite are

irregular, the self-setting particle-stabilized porous ce-

ramic panel would have intermediate characteristics be-

tween an open-cell pore structure and a closed-cell pore

structure, which means that further coating processes

could be adopted if needed.

As far as the authors know, no research group has

studied a self-setting particle-stabilized porous ceramic

panel composited with a radar absorbing material, al-

though several recent reports show the feasibility of

self-setting particle-stabilized porous ceramics using

calcium aluminate cement [26–28]. Therefore, the aim

of this study was to clarify whether a porous ceramic

panel composited with carbon can be prepared by a

self-setting particle-stabilized method without a con-

ventional sintering process.

II. Material and methods

2.1. Sample preparation

Zeolite 13X (Molecular sieve 13X, Sigma-

Aldrich, USA) and propyl gallate (3,4,5-

(HO)3C6H2CO2CH2CH2CH3, 98% pure, Sigma-

Aldrich, USA) were used as pore formers. We added

40 ml of distilled water, 0.6 g of propyl gallate, 20 g

of commercial calcium aluminate cement (KS L 5201

standard, Tongyang Cement, Korea), and carbon (Nano

carbon, Sigma-Aldrich, USA) to 20 g of the zeolite

13X. Optionally, 1 g of lithium carbonate (99% pure,

Sigma-Aldrich, USA) was also added. We had deter-

mined by preliminary experiments that the maximum

amount of carbon to avoid carbon agglomeration is

0.2 wt.% and the minimum amount of carbon to affect

the reflection loss is 0.04 wt.%.

Foaming was carried out using a direct-driven motor

at a speed of 1000 rpm. The slurry was poured into an

acrylic mold and allowed to set for 48 h at room tem-

perature for self-setting. For comparison, 40 g of com-

mercial calcium aluminate cement and 40 ml of distilled

water were used for the preparation of a self-setting ce-

ramic panel.

2.2. Characterization

The pore characteristics of the self-setting particle-

stabilized porous ceramic panel were investigated

by scanning electron micrography (JSM-5800, JEOL,

Japan), and mercury porosimetry (Autopore IV 9510,

Micromeritics, USA). The compressive strengths were

measured by a uni-axial mechanical tester (Instron

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4206, Instron, USA) with 10 mm × 10 mm × 10 mm

specimen dimensions, and a crosshead speed of

0.5 mm/s. To measure the complex permittivity of

the self-setting particle-stabilized porous panel at X-

band, an Agilent N5230A analyser (PNA-L Vector Net-

work Analyzer) was used. The dimensionless permit-

tivity was obtained from the scattering parameters for

reflected and transmitted microwaves over the 8.2–

12.4 GHz range using Agilent 85071E (Materials Mea-

surement Software), which employs the Nicolson-Ross-

Weir method. In detail: the complex permittivity, per-

meability, and reflection loss (electromagnetic wave ab-

sorbing performance) of the samples with dimensions

of 150 mm × 150 mm × 10 mm were measured by a free

space measurement system which consists of transmit-

ting and a receiving spot-focusing horn antennas, a net-

work analyser, and a sample holder. Compared to tradi-

tional measurements of electromagnetic properties such

as coaxial, waveguide or cavity resonance methods, the

contactless and nondestructive free space measurement

system is highly effective for brittle samples like the

open-pore ceramic specimens. The material parameters

were determined from the measured transmitted and re-

flected scattering parameters.

Note that, hereinafter, CP denoted the self-setting

ceramic panel, and PPCP denoted that self-setting

particle-stabilized porous ceramic panel.

III. Results and discussions

The PPCP panel after 48 h of setting time and the

acryl mould that was used are shown side by side in Fig.

1. Although the authors used a 15 cm × 15 cm square

acryl mould, the shape and size of the sample have no

restrictions because of its self-setting ability.

Prior to analysing the PPCP samples, a CP was pre-

pared for comparison. A typical scanning electron mi-

croscope (SEM) image of the CP sample after 48 h of

setting time is shown in Fig. 2a. The CP had the typ-

ical microstructure of the hardened calcium aluminate

cement. In preliminary experiments, a needle-like et-

tringite phase was formed as a result of the reaction

of calcium aluminate with the calcium sulphate that is

commonly present in the microstructure of the hard-

ened calcium aluminate cement, as seen in Fig. 2b. It is

Figure 1. Image of the PPCP after 48 h of self-setting timeand the acryl mould that was used

well known that the ettringite phase does not contribute

to strength [29]. Moreover, it should be noted that the

voids within the strut walls have a detrimental effect

on material strength. Hence, the presence of an ettrin-

gite phase in the PPCP would prevent it from having

adequate strength for handling. Therefore, lithium car-

bonate was added to minimize the ettringite phase [28].

The authors expected that the crystallization of ettringite

could be inhibited by accelerating the hydration process

of the calcium aluminate cement.

Typical SEM images of the PPCP after 48 h of set-

ting time are shown in Figs. 2c and 2d. The panel con-

tained macropores, with a size range of approximately 5

to 50 µm, formed by the zeolite 13X particles in the cal-

cium aluminate cement matrix that were irreversibly ad-

sorbed at the liquid-gas interfaces. If an ettringite phase

had been generated, the majority of these macropores

would have decomposed during the hydration process

in the presence of lithium carbonate, similar to the case

of barium carbonate [30]. The macropores were highly

inter-connected with neighbouring macropores through

voids within the strut walls. The microstructure of the

PPCP sample shows voids within the strut walls with a

few ettringite needles among the zeolite 13X particles.

Therefore, the microstructure of the panel is typical of

the particle-stabilized direct foaming method.

The particle-stabilized direct foaming method is in-

trinsically based on the idea that particles can be used

to adsorb irreversibly on the surfaces of gas bubbles to

stabilize wet foams. Upon adsorption, particles lower

down the overall free energy of the system by replacing

part of the highly energetic gas-liquid interfacial area

with less energetic interfaces [31]. Therefore, if the par-

ticles are uniform and spherical, the surfaces of the gas

bubbles are fully covered with a single layer of parti-

cles. If the particles are irregular, the surfaces of the

gas bubbles are covered with several layers of particles.

In this study, because zeolite 13X particles and calcium

aluminate cement particles are non-uniform, there were

many inter-particle voids induced by the irregular par-

ticles. Thus, the surfaces of the gas bubbles were cov-

ered with as many particles as possible, which is ener-

getically favourable. Therefore, it was observed that the

PPCP had intermediate characteristics between an open-

cell pore structure and a closed-cell pore structure, as

shown in Figs. 2c and 2d.

Figure 3 shows the pore size distributions of the

PPCP and CP samples. The PPCP shows a peak at ap-

proximately 5 to 50 µm and another peak at approx-

imately 100 to 1000µm. The former corresponds to

inter-particle voids between the zeolite 13X particles,

whereas the latter corresponds to the voids inside the

strut walls, throats, or entrance openings of spherical

macropores because mercury enters the macropores at

a pressure determined by the entrance size rather than

by the actual spherical pore size [32]. The CP shows

few peaks because of the lack of macropores inside the

microstructure, as shown in Fig. 2a.

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J.-H. Ha et al. / Processing and Application of Ceramics 12 [1] (2018) 86–93

(a) (b)

(c) (d)

Figure 2. SEM images of: (a,b) the CP after 48 h of self-setting time, (c,d) the PPCP after 48 h of self-setting time

Figure 3. The pore size distributions of the PPCP and the CPsamples

The authors could not expect good radar-absorbing

properties of a self-setting ceramic panel regardless of

the presence of macropores, unless carbon was added

as a radar-absorbing material. Therefore, carbon was

added to a self-setting particle-stabilized porous ce-

ramic panel. Typical SEM images of the PPCP compos-

ited with 0.2 wt.% of carbon after 48 h of self-setting

time are shown in Figs. 4a and 4b. The PPCP compos-

ited with 0.2 wt.% of carbon contained macropores with

a size range of approximately 5 to 50µm formed by the

zeolite 13X particles in the calcium aluminate cement

matrix that were irreversibly adsorbed at liquid-gas in-

terfaces as seen with the PPCP without carbon. How-

ever, it was difficult to observe the presence of carbon

because the amount added was very low.

If the density is high (thus increasing the material

weight) or if the surface area is not high enough to ef-

fectively impregnate material (thus reducing its radar-

absorbing properties), this process would not be ac-

ceptable for radar-absorbing applications, even though

the authors could obtain a PPCP as a rigid mono-

lith. Therefore, the pore volumes of the PPCP, the

CP and zeolite 13X (pore former) were measured us-

ing Barrett-Joyner-Halenda (BJH) analysis, and the

results are shown in Fig. 5a. Figure 5b shows the

Brunauer-Emmett-Teller (BET) surface area and den-

sity of each material. The density (1.80 g/cm3) and the

BET surface area (359.67 m2/g) of the PPCP are be-

tween those of a CP (3.16 g/cm3, 1.02 m2/g) and ze-

olite 13X (1.43 g/cm3, 729.10 m2/g). In the literature,

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(a) (b)

Figure 4. (a) SEM of the self-setting particle-stabilized porous ceramic panel composited with 0.2 wt.% of carbon after 48 h ofself-setting time and (b) magnification of its macropore morphology

(a) (b)

Figure 5. (a) The BJH adsorption pore volume and (b) the BET surface areas and bulk densities of the PPCP, the CP and thezeolite 13X powder (pore former)

self-standing zeolite monoliths such as faujasite zeolite

(63.31 m2/g, and 19.6 MPa) [33], NaY (97.81 m2/g, and

57.60 MPa) [34], and Zeolite A (30 m2/g, and 0.7 MPa)

[35] showed reduced BET surface areas, although the

compressive strengths of these monoliths were signif-

icantly enhanced. Therefore, it is noteworthy that the

PPCP, prepared in this study maintained its BET sur-

face area (359.67 m2/g), following the rule of mixtures

with calcium aluminate cement, and has an acceptable

compressive strength under non load-bearing conditions

(1.80 MPa).

Figure 6a shows the permittivities of the PPCP com-

posite with 0.04 wt.% and 0.2 wt.% of carbon. The

higher the carbon weight percent is, the greater the

permittivity is. This might be explained by the con-

ductive behaviour of the carbon particles inserted into

the porous ceramic matrix. This trend corresponds well

with the previously published data on carbon-based

radar-absorbing materials [36,37]. Some of the incident

radar waves will be absorbed by the radar-absorbing

material; the rest will start propagating through the

medium with lower intensity than the incident wave

[38]. The open-cell pore structure of a PPCP enables

the free movement of waves inside the medium, and the

radar-absorbing material (in this study, carbon) present

inside the medium absorbs some of the radar waves.

The reflection losses of both the PPCP and the PPCP

composited with 0.2 wt.% of carbon were measured in

the range of 8.2 to 12.4 GHz (X-band), as shown in Fig.

6b. For a radar-absorbing material, the reflection loss is

calculated from the complex permittivity and the per-

meability at a given frequency and absorber thickness

according to transmission line theory. The measured re-

flection loss of the PPCP composited with 0.2 wt.% of

carbon approaches −11.16 dB at 12.4 GHz for a 7.0 mm

thickness, which corresponds to ≤ −10 dB reflection

loss (i.e., more than 90% absorption of the incoming

radar wave).

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J.-H. Ha et al. / Processing and Application of Ceramics 12 [1] (2018) 86–93

(a) (b)

Figure 6. (a) Permittivities of PPCPs composited with 0.04 wt.% and 0.2 wt.% of carbon. (b) Reflection losses of PPCPscomposited with 0.2 wt.% of carbon and without addition of carbon

These data are comparable with those of the carbon-

based radar-absorbing materials reported in the lit-

erature, such as carbon nanotube-based composites

(98.54% absorption at 11.1 GHz with a thickness of

2.0 mm [12], 98.4% absorption at 8 GHz with a thick-

ness of 4.0 mm [11]), and carbon-based composites

(99.5% absorption at 11.5 GHz with a thickness of

4.0 mm [13]). Compared to the above mentioned liter-

ature data, the reflection loss of the PPCP composited

with 0.2 wt.% of carbon was acceptable, because a self-

setting particle-stabilized porous ceramic panel is low-

cost (the inexpensive raw materials), lightweight (the

particle-stabilized pore structure), and easy to prepare

(the self-setting fabrication method).

The reason behind the better absorption of the PPCP

composited with 0.2 wt.% of carbon over a PPCP is the

efficient combination of porous ceramic matrix and car-

bon particles. Usually, the radar-absorbing properties of

dielectric materials are considered to be related to the

efficient complementarity between the imaginary part

and the real part of the permittivity in the materials

[39]. Carbon is a conductive material with a large real

part and imaginary part of its permittivity, hence, the

poor impedance match results in weak absorption of the

incoming electromagnetic wave. Therefore, controlling

the amount of carbon added to a porous ceramic matrix

is an effective way to tailor the permittivity and improve

the impedance match. In this work the PPCP compos-

ited with 0.2 wt.% of carbon may possess better coop-

erative interaction than the other samples, resulting in

better radar-absorbing properties.

In this study, the authors investigated the feasibil-

ity of a self-setting particle-stabilized porous ceramic

panel as a low-cost, lightweight radar-absorbing mate-

rial. Because radar-absorbing properties can generally

be tuned by varying the thickness of the panel, the thick-

ness is very important when dealing with electromag-

netic waves. Also, the self-setting method can provide a

PPCP with a uniform surface and thickness. The effects

of the amount of added carbon, and the thickness varia-

tion of a PPCP on the radar-absorbing properties remain

important issues for further research.

IV. Conclusions

In summary, a self-setting particle-stabilized porous

ceramic panel contained macropores formed by zeo-

lite 13X particles that were irreversibly adsorbed at the

liquid-gas interfaces. The density of the panel was re-

duced to that of typical lightweight porous ceramics be-

cause of the highly inter-connected macropores, while

obtaining an acceptable BET surface area for any further

coating. Furthermore, the self-setting particle-stabilized

porous ceramic panel can be prepared without cracks or

defects induced by a delicate and costly sintering pro-

cess and free of shape and size limitations. Therefore,

combination of the particle-stabilized direct foaming

method (using zeolite 13X and propyl gallate) and the

self-setting method (using calcium aluminate cement

and lithium carbonate) can provide an easy, inexpen-

sive, and rapid approach to fabricating a highly porous

ceramic with intermediate characteristics between an

open-cell pore structure and a closed-cell pore struc-

ture. Also, when 0.2 wt.% carbon was added to a self-

setting particle-stabilized porous ceramic panel to fab-

ricate a 7 mm thick composite, the maximum reflection

loss was −11.16 dB at 12.4 GHz. These findings show

the feasibility of using a self-setting particle-stabilized

porous ceramic panel composited with carbon in poten-

tial radar-absorbing applications.

Acknowledgments: We acknowledge the financial sup-

port of the Ministry of Science and ICT through

the National Research Foundation of Korea (NRF-

2017K1A3A1A39092683).

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